1. Field of the Invention
The present invention relates to a field emission device and more particularly, to a field emission device capable of automatic compensation of the distribution deviation of emitted electrons from a cathode.
2. Description of the Prior Art
Conventionally, various types of field emission devices have been developed, typical examples of which were reported by C. A. Spindt et al. in the paper, Journal of Applied Physics, Vol. 47, No. 12, pp. 5248-5263, published in December 1976, and by H. F. Gray et al. in the paper, 1986 IEDM Technical Digest, pp. 776-779, published in 1986.
An example of the conventional field emission devices is shown in FIG. 1, which includes a semiconductor substrate 131. An insulating layer 132 is formed on the substrate 131. A conductive layer 133 is formed on the insulating layer 132. The conductive layer 133 serves as a gate electrode and has circular windows 134 uncovering the surface of the underlying insulating layer 132. The windows 134 are arranged in a matrix array. The insulating layer 132 has penetrating circular holes (not shown) formed at the locations corresponding to the windows 134, respectively, thereby exposing the surface of the substrate 131.
Conical cathodes 135, which are made of a conductive metal such as molybdenum (Mo), are formed on the exposed surface of the substrate 131 in the holes of the insulating layer 132, respectively. Each of the cathodes 135 has a shape of a sharp-pointed cone. The tips of the cathodes 135 are located in the vicinity of the interface of the gate electrode 133 and the insulating layer 132.
When a positive electric potential is applied to the gate electrode 133 with respect to the conical cathodes 135 in a vacuum atmosphere, electrons (not shown) are emitted or extracted from the vicinity of the tips of the cathodes 135 due to the "field emission" phenomenon. The emitted electrons move upward in the space near the gate electrode 133, traveling toward an anode (not shown).
The condition for the field emission phenomenon of the electrons is determined according to the shape of the cathodes 135 and the distances between the gate electrode 133 and the corresponding cathodes 135.
With the conventional field emission device shown in FIG. 1, the overall emission direction of the electrons is dependent upon (a) the electric-field distribution in the vicinity of the tips of the conical cathodes 135, (b) the surface state (for example, the work function and electron density) of the cathodes 135 in the vicinity of their tips, and (c) the surface configuration (for example, surface irregularities) of the cathodes 135 in the vicinity of their tips.
If the shape of each cathode 135 is accurately conical, the surface state and configuration of the cathode 135 are axially symmetric with respect to its central axis penetrating the tip, and the distance of the cathode 135 from the opposing inner edge of the gate electrode 133 is axially symmetric with respect to the central axis, the emitted electrons will travel from the tip through a conical region with a solid angle of approximately 40.degree..
However, if the above-described axial symmetry of either the shape, or the surface state, or and the surface configuration is collapsed, the central or symmetric axis of the distribution of the emitted electrons will deviate from its original position. This symmetric-axis deviation causes a problem that aberration tends to occur in the case where the emitted electrons are collected by an electron lens or lenses (not shown) or that deviation of the overall emission direction of the electrons from an intended direction tends to occur even for the case where no electron lens is used.
Further, the above symmetric-axis deviation causes another problem that the emitted electrons tend to stream into or enter the gate electrode 133, thereby consuming surplus electric power. This surplus power consumption heats the neighboring regions of the gate electrode 133 with the cathodes 135 to generate some gaseous material. The generated gaseous material leads to the insulation breakdown phenomenon. Therefore, a "discharge current" tends to flow between the gate electrode 133 and the cathodes 135, destroying or damaging the gate 133 and/or the cathodes 135.
At this stage, a part of the emitted electrons tend to directly or indirectly enter the insulating layer 132, thereby charging-up the insulating layer 132. Thus, there arises a problem that the insulating characteristic of the field emission device degrades. In other words, a leakage current tends to flow between the gate electrode 133 and the substrate 131. This results in an unstable potential difference between the gate electrode 133 and the substrate 131 (i.e., the cathodes 135).
A typical example of the above-described destruction of the gate electrode 133 and/or the cathodes 135 due to the discharge current is the short-circuit-induced destruction, where the gate electrode 133 and at least one of the cathodes 135 are electrically connected to be destroyed due to short-circuit.
To solve the problem of the short-circuit-induced destruction, the following improvement was developed as disclosed in the Japanese Non-Examined Patent Publication no. 4-284324 published in 1992.
In this improvement, a gate electrode is divided into a plurality of electrode elements; an unsoluble main element and soluble branch elements located at respective cathodes. When short-circuit occurs between the gate electrode and any one of the cathodes, the corresponding one of the branch elements of the gate electrode is solved or melted by heat due to the short-circuit, thereby being separated from the remainder. Thus, the normal operation of the remaining cathodes is surely kept even after the short-circuit.
However, the improvement disclosed in the Japanese Non-Examined Patent Publication no. 4-284324 cannot solve the previously-explained problem caused by the deviation of the symmetric axis of the emitted electron distribution.